C malloc segmentation fault struct

Question

In the textbook my teacher provided us, there is this C code's sample, which when I try to run gives a Segmentation Fault error:

const celula *nulo = NULL;
typedef char informacao;
typedef celula *noh;
typedef celula *arvore;
struct celula {informacao info; noh pai; noh fesq; noh idir;};

...

typedef struct celfloresta celfloresta;
typedef struct celfloresta *floresta;
typedef struct celfloresta *posicfloresta;
struct celfloresta {arvore elem; celfloresta *prox;};

...

void FormarListaNohs(){
    floresta p = (floresta)malloc(sizeof(celfloresta));
    p->elem->info = '3';
}

...

Why does the line p->elem->info = '3'; give segmentation fault here?

regarding: const celula *nulo = NULL; This is using the celula before it is defined. Suggest moving that statement to after the definition of the struct — user3629249
– user3629249, Commented Dec 18, 2019 at 4:14
OT: regarding: floresta p = (floresta)malloc(sizeof(celfloresta)); 1) in C, the returned type is void* which can be assigned to any pointer. Casting just clutters the code. Suggest removing the cast. 2) always check (!=NULL) the returned value to assure the operation was successful. — user3629249
– user3629249, Commented Dec 18, 2019 at 4:16
OT: regarding: typedef struct celfloresta *floresta; and typedef struct celfloresta *posicfloresta; and typedef celula *noh;` and typedef celula *arvore; It is a very poor programming practice (and very misleading to the human reader of the code) to hide pointers in a typedef. — user3629249
– user3629249, Commented Dec 18, 2019 at 4:19

Saucy Goat · Accepted Answer · 2019-12-17 19:20:28Z

2

elem is a pointer. You need to allocate memory for it:

p->elem = malloc(sizeof(arvore));

answered Dec 17, 2019 at 19:20

Saucy Goat

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medmik · Accepted Answer · 2019-12-17 19:34:35Z

0

Basically malloc return a void pointer then to cast is should use a variable of type pointer an example :

int *p = malloc(sizeof(int))
struct s_list *l = malloc(sizeof(struct s_list))

then you can dereference the pointer for example

l->data = 12;

answered Dec 17, 2019 at 19:34

medmik

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